Characterization of European Plasmopara viticola isolates with reduced sensitivity to cymoxanil

Abstract

Cymoxanil has been used for over 30 years to control grape downy mildew (Plasmopara viticola) in European vineyards, prevalently in mixture with other fungicides active on this disease. In the 1990’s cases of P. viticola resistant to cymoxanil were detected using a leaf disc assay. In this study, we establish that the presence of only 1 % of resistant isolates in a P. viticola population will allow the detection of cymoxanil resistance in the leaf disc assay. A poor correlation (R = 0.194) was observed between the leaf disc assay and a whole- plant test for 38 P. viticola field populations collected in 2004. Over 60 % of these populations were characterized as fully sensitive in a whole-plant assay compared to 10 % in the leaf disc assay. Five P. viticola field isolates resistant to cymoxanil reverted to full sensitivity after six to nine transfers to untreated vines, indicating that cymoxanil resistance in P. viticola is unstable. Two European P. viticola populations sensitive to cymoxanil became resistant when transferred 12–14 times on vines treated with cymoxanil. In contrast, two populations originating from the USA and three monozoospore isolates from France retained full sensitivity to the fungicide after 13 cycles on cymoxanil-treated plants. Whole-plant experiments were conducted in the laboratory to compare the efficiency of spray programs to delay the development of cymoxanil resistance. Whereas the continuous use of cymoxanil alone quickly selected for resistance, the mixture of cymoxanil and folpet applied either continuously or in strict or block alternation effectively prevented the development of resistance over 10 generations of the fungus. These results demonstrate that resistance to cymoxanil in P. viticola can be managed with appropriate spray programs.